1. Field of the Invention
The present invention generally relates to electrical shock prevention and, more particularly, to a method and apparatus for detecting leakage current on a two wire power line.
2. Background Description
In the past, the railroads have typically operated trains having only a single air line extending the length of the train. This air line was used for both providing a source of compressed air and a medium for propagating braking signals. While this system has been used extensively in the past, it has several drawbacks. Signaling via air pressure messages propagating through the air line has a limited propagation speed. For example, for a 150-car freight train, it may take fifteen seconds or more for a braking message to reach the 150th car, thereby delaying the full application of the rail car brakes and consequently extending the distance required to stop the train. In recent years, the Association of American Railroads (AAR) and individual railroads have investigated using electronically controlled pneumatic (ECP) brake systems. These systems typically use electronic messages on a power line extending the length of the train to activate the brakes on each car because the electronic signal propagation velocity is theoretically limited only by the speed of light or about 983,571,056 feet per second in a free space environment. However, in a cable, the speed of electronic signal propagation may slow to 60 percent of the speed of light in a vacuum, which still would be about 590,000,000 feet per second. For a typical freight train consisting of 150 cars each approximately 60 feet long, a train length could be approximately 9,000 feet. An electronic signal in a cable will travel the length of the train in only about 15 micro seconds while a pneumatic signal is limited to the speed of sound in air or about 1,130 feet per second. However, in a pipe with numerous couplings, turns, and other restrictions, the pneumatic signal propagation may slow to between 600 and 900 feet per second. At 600 feet per second, this pneumatic signal will require about 100 milliseconds to propagate through each car or about fifteen seconds to propagate the length of the train. The ECP brake system allows for nearly instantaneous activation of the railcar brakes along the entire length of the train. These ECP systems have been tested in the field and now are being considered for definition in an AAR specification. Persons skilled in the art are aware of the existing AAR efforts and the numerous tests of ECP and ECP-like field tests which have occurred.
Cable based ECP brake systems for railroads utilize a two wire cable that carries both digital communication signals and system power. The system power used is presently 230 Volts DC, but other AC or DC voltages could be used. The two-wire power system is floating relative to earth ground and the American Association of Railroads (AAR) approved connectors for the system do not support the addition of a third wire for a safety ground. The metal body of railroad cars, the tracks, the inter-car couplers form a somewhat intermittent connection to earth ground and to each other. Given that the body of any railcar may be isolated from earth and other railcars at any given time, it is possible that a short circuit between one of the conductors of the train line and the car body could result in a dangerous voltage being present on the body of the car. Conventional Ground Fault Interruption (GFI) devices do not work well in this application because they are triggered by the digital communication signals present on the same wires.